Phosphoric acid is a primary ingredient for the production of various phosphate products, including diammonium phosphate fertilizer (DAP); merchant-grade phosphoric acid (MGA); superphosphoric acid (SPA), and other various fertilizer and industrial-grade products. The phosphoric acid is made from the reaction of mined phosphate rock with sulfuric acid (H2SO4), for the most part, to produce a phosphoric acid solution and a by-product gypsum material. Phosphoric acid can also be produced via the reaction of phosphate rock with other acids such as hydrochloric acid and nitric acid, but the majority of the production is with sulfuric acid.
Since the phosphate rock is a mined material, it generally contains a number of associated impurities. Typically the rock can contain levels of iron, aluminum, magnesium (the so-called “minor elements”), as well as limestone, excess silica, traces of uranium and rare earths, and appreciable levels of fluoride components. To some extent, many of these impurities dissolve into phosphoric acid during the reaction of the phosphate rock with the acid source.
Phosphate rock itself is quite variable depending on its source. Some rocks are quite low in associated impurities and in general do not required any extra levels of beneficiation to produce the desired end-products. Other rock sources can contain relatively high levels of impurities and in many cases these impurities can make it difficult, if not impossible, to produce specification products.
Further as various rock sources are mined the typical trend is to mine the highest quality material first, and then over time mine the lower grade ore. In some cases, even though there is phosphate rock still available at a mine, the contained impurities are too high, and the rock cannot be chemically processed using the conventional technologies, thus the low-grade ore becomes essentially valueless. This, in turn, results in less than full recovery of the potentially useable phosphate ore.
Since the presence of impurities, especially the so-called “minor elements”, can be detrimental to the phos-acid producer's ability to produce specification products, as well as diminish the potential phosphate rock that can be recovered from a given mine site, it would be desirable to have a method that can be used to reduce the dissolved impurities that are extracted into the acid and remove these impurities in such a manner that associated phosphoric acid losses are minimized and any phosphoric acid dilution is also held to a minimum. This would result in a significant increase in the quality of acid that could be produced by the phosphoric acid operation even with lower grade phosphate rock sources. Further, this would also greatly expand the potential phosphate rock reserve base for the phosphate mining operations and allow for better overall utilization of resources from a given developed mine site.